You've got about a half inch of clearance in front, and a lot in the rear. I went from 255s to 295s in the back with no problem. The picture is of 235 35 18 Hoosier A6's with a 5mm spacer up front. Including the spacer, I had about 1/4 to 3/8" clearance from that pesky upper control arm bolt (some folks have done a little grinding on it). If you go from the stock 40, to 35 height, you can gain some width. And remember, all tires are a little different. Good luck.
Mike